1. Field of the Invention
The present invention relates to a temperature sensing circuit, and more particularly, to a temperature sensing circuit in a cooking appliance that uses one temperature sensor to accurately measure temperatures of a low temperature range and a high temperature range to aid in the precision controlling of the cooking appliance, and to a controlling method of the temperature sensing circuit.
2. Description of the Related Art
In general, a cooking appliance uses a temperature sensor for measuring the temperature inside a heated cooking compartment.
Thermistors are widely used as temperature sensors in cooking appliances, and can be divided into negative temperature coefficient (NTC) type thermistors whose resistance values decrease with an increase in temperature, and positive temperature coefficient (PTC) type thermistors whose resistance values increase with an increase in temperature.
A cooking appliance uses one thermistor that displays a change in its resistance value to reflect a change in temperature in the cooking appliance. Also, the change in the resistance value is amplified using a temperature sensing circuit, to estimate the temperature inside the cooking appliance. Thus, the resistance value detected by the temperature sensor is amplified according to the gain in an amplifier to change the gain of the resistance value, after which the resistance value is outputted to a microprocessor. Then, a temperature table of a low temperature range is used when the gain of the resistance value is high, and a temperature table of a high temperature range is used when the gain is low.
Therefore, in the case of a low temperature range, because the slope of the sensor output voltage according to the inner temperature of the cooking appliance becomes greater compared to a high temperature range, a high resolution can be obtained.
However, in most cases where food is cooked, because a cooking appliance is heated to a high temperature, a temperature table of a high temperature range is used in performing the cooking. Only in special cases, such as a rising or keep warm cycle, is a temperature table of a low temperature range used. That is, the type of temperature table to be used is predetermined according to the type of cooking to be done.
Thus, when a cooking appliance operates, although the temperature inside the cooking chamber at the start of cooking is low, because a high temperature table is used based on a cooking temperature preset according to type of food, accurately maintaining a constant temperature is difficult. That is, because a temperature table of a high temperature range with a low gain of a resistance value is used in most cases, the temperature table of a low temperature range is hardly used.